This research project focusses on cell cycle control as a major determinant of chemosensitivity. We aim to utilize cell cycle control as a basis for the development of new chemotherapeutic stratagems to selectively kill tumor cells. Research is oriented towards the molecular dissection of checkpoint control systems in human cells and to uncover defects in neoplasia. Our plan is to precisely map these control systems from the point where DNA damage or unreplicated DNA is sensed to the response elements that arrest cell cycle progression. We are also searching for molecular indicators to predict the fidelity of checkpoint control. Molecular dissection of the G2 checkpoint in human lymphoma cells revealed that the cdc2 kinase activity is suppressed by inhibitory phosphorylations. Failure to remove these phosphates correlated with suppression of the cdc25C phosphatase. Cdc25C activity is up-regulated by hyperphosphorylation and this did not occur in G2 arrested cells. We are now investigating the kinase/phosphatase couple that regulates cdc25C in response to DNA damage. We have found that G2 arrest does not inhibit the activation of cyclin A/cdk2 complexes while cyclin A/cdc2 and cyclin B1/cdc2 complex activation is suppressed. We suggest that activation of cyclin A/cdc2 complexes might act together with cyclin B1/cdc2 complexes to promote mitosis. We have also commenced a characterization of the G1 checkpoint in human cells with emphasis on the role of the tumor suppressor gene, p53 in G1 arrest induced by DNA damage. We are currently characterizing the interaction of p53 with the G1/S phase cyclin-dependent kinases. We are examining the actions of kinase/phosphatase inhibitors on cell cycle control to search for modulators that could enhance the antitumor activity of chemotherapeutic agents. For example, we are searching for non-toxic, non-DNA damaging agents that prolong the half- life of p53. Such agents will be used to arrest cells with normal p53 in G1. Tumors with defective p53 will then be selectively killed with cytotoxic agents specific for S and M phase.